Additional resources from the Raylib repo.

Examples/resources/shaders/glsl120/outline.fs

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+#version 120
+
+// Input vertex attributes (from vertex shader)
+varying vec2 fragTexCoord;
+varying vec4 fragColor;
+
+// Input uniform values
+uniform sampler2D texture0;
+uniform vec4 colDiffuse;
+
+uniform vec2 textureSize;
+uniform float outlineSize;
+uniform vec4 outlineColor;
+
+void main()
+{
+    vec4 texel = texture2D(texture0, fragTexCoord);   // Get texel color
+    vec2 texelScale = vec2(0.0);
+    texelScale.x = outlineSize/textureSize.x;
+    texelScale.y = outlineSize/textureSize.y;
+
+    // We sample four corner texels, but only for the alpha channel (this is for the outline)
+    vec4 corners = vec4(0.0);
+    corners.x = texture2D(texture0, fragTexCoord + vec2(texelScale.x, texelScale.y)).a;
+    corners.y = texture2D(texture0, fragTexCoord + vec2(texelScale.x, -texelScale.y)).a;
+    corners.z = texture2D(texture0, fragTexCoord + vec2(-texelScale.x, texelScale.y)).a;
+    corners.w = texture2D(texture0, fragTexCoord + vec2(-texelScale.x, -texelScale.y)).a;
+
+    float outline = min(dot(corners, vec4(1.0)), 1.0);
+    vec4 color = mix(vec4(0.0), outlineColor, outline);
+    gl_FragColor = mix(color, texel, texel.a);
+}